I add a new page and make it a sub-page in the Pages menu.
I Add the page as a subpage to the menu in Appearance/Menus.
Once I click to save this menu, the sub-page 'springs' back to the left and is a page on the top-level again.
I.o.W. I cannot create a menu with a sub-menu.
Do I do something wrong, coz the theme should be able to have at least 3-level-menus.
Thanks for your help.
Thomasthe mad bean

- Turning off all your plugins. Is the issue gone? If it is, turn them back on one-by-one to find the conflict.

- Switching to a default theme like Twenty Fourteen or Twenty Thirteen and see if your menus work as expected. You may need to reset your widget and other theme-specific settings afterwards, so you might want to make a note of them.

- Using a different browser. If it happens only in one web browser (like Firefox or Chrome) then try deactivating all of that browsers add-ons or extensions. If the problem goes away, then you know an add-on is to blame and you can try turning them back on one by one to determine which one is at fault.